It is well known in the art of drilling subterranean wells to form a parent wellbore into the earth and then to form one or more wellbores extending laterally therefrom. Generally, the parent wellbore is first cased and cemented, and then a guiding tool is positioned in the parent wellbore atop an anchor structure locked into place in the parent wellbore casing. The guiding tool includes a sloped surface disposed to guide a cutting mill lowered into the wellbore. More particularly, the tool, often referred to as a whipstock, deflects the cutting mill so that a blade of the cutting mill engages the casing, thereby permitting a window to be milled in the casing and cement. Milling the side wall window in the parent wellbore casing facilitates the subsequent addition of a lateral wellbore thereto. Directional drilling techniques may then be employed to direct further drilling of the lateral bore through the milled window as desired.
The lateral bore is then cased by inserting a tubular liner from the parent bore, through the window previously cut in the parent bore casing and cement, and then into the lateral bore. In a typical lateral bore casing operation, the liner extends somewhat upwardly into the parent bore casing and through the window when the casing operation is finished. In this way, an overlap is achieved wherein the lateral bore liner is received in the parent bore casing above the window.
In some milling system, rather than a whipstock, a mandrel having guide surface may be employed to urge the mill blade into contact with the casing. Thus, a milling system may generally include a mandrel that carries a cutting mill with carriage mounts disposed on either side of the cutting mill. A tubular mill housing has a mill housing opening that forms elongated tracks thereon. Each track has a sloped section and an elongated flat section that extends along a substantial portion of the length of the mill housing. During cutting, the mandrel is moved relative to the mill housing. Specifically, the carriage mounts slide along elongated the tracks. The sloped part of the tracks allows the cutting mill to progressively engage the casing to begin a cut. Once the casing is engaged and an initial hole is milled, the cutting mill is moved along the elongated flat section of the ramp, thereby milling an elongated window in the casing. The cutting mill inner diameter (ID) access dimensions are limited by the dimensions of the mill housing. The current system is limited in this way due to a throat at the top of the mill housing which limits the maximum mill driveshaft diameter and the fixed mill guide limits the maximum diameter of the mill blade and driveshaft.
Each of these structures, however, has one or more disadvantages which make its use inconvenient or uneconomical. Some of these disadvantages include inaccurate positioning and orienting of the window opening to be cut, complexity in setting and releasing the mill, undesirable torque-created rotational shifting of the mill, and the inability to control the effects of weigh on the mill, particularly in offshore environments where heave can quickly alter the weight on the mill, leading to damage of the mill.